Uzbekistan. His generals raided Iran and
Russia.
Genghis Khan began his last battle
against China in 1226. He died on
August 18, 1227. Mongols then controlled
land from the China Sea to the
European part of Russia. Genghis
Khan’s sons and grandsons expanded
the Mongol Empire even more.
#More to explore
Mongol Empire • Mongolia
Geography
Geography is a science that deals with
Earth’s surface. People who study geography
are called geographers. Geographers
are interested in Earth’s physical
features, such as mountains, deserts,
rivers, and oceans. They are also interested
in the ways that people affect and
are affected by the natural world.
Branches of Geography
Geography can be divided into two
branches: physical geography and
human geography. Physical geographers
observe, measure, and describe Earth’s
surface. They study how landforms
develop and how they change. They
look at how different landforms affect
climate. They also study how people
change the land through such activities
as building cities, digging mines, and
clearing forests.
Human geography focuses on where
people live, what they do, and how they
use the land. Human geographers might
study why cities and towns develop in
certain places. Others study the cultures
of different peoples, including their customs,
languages, and religions.
How GeographersWork
Geographers use a number of methods
and tools in their work. The simplest
method is going out to a place to
observe and gather information. This is
called fieldwork. Geographers use photographs
taken from the air to see things
that they cannot see from the ground.
They also depend on spacecraft called
satellites to take photographs and collect
information from far above Earth.
A map is a helpful tool in the study of geography.
Temujin was
named after
an enemy his
father had
defeated.
96 Geography BRITANNICA STUDENT ENCYCLOPEDIA
Geographers use computers to help
them make sense of the information
they receive.
The main tool used by geographers is
the map. People who make maps are
called cartographers. Geographers use
maps to display the information they
have gathered. Topographic maps show
natural land formations, such as the
huge landmasses called continents.
Geographers also use maps to show how
people have divided the land. Political
maps show the boundaries of countries
and other divisions.
#More to explore
Climate • Continent • Earth • Map and
Globe
Geology
Geology is the study of the physical
features and history of Earth. Scientists
who work in geology are called
geologists.
Geology is an important science for
many reasons. It answers questions
about how Earth came to have its
present shape and form. It is useful for
finding important materials in Earth’s
crust, such as oil. Geology is also helpful
for predicting earthquakes and other
natural hazards.
Branches of Geology
Geologists work in many different
branches of geology. Some geologists
study rocks and minerals and how they
formed. Some study the structure of
Earth and the different forces and
actions inside it. Some study how landforms,
such as mountains, on Earth’s
surface develop and change. Some
geologists, called paleontologists, study
fossils. (Fossils are the traces of prehistoric
plants and animals.) Still other
geologists work out how humans can use
Earth’s resources without harming the
environment.
Geologic Time
Most geologists agree that Earth started
forming about 4.6 billion years ago.
After about 700 million years it had
developed a solid crust. Geologists use
the words geologic time to describe the
vast amount of time that has passed
since then. They divide this time into
three broad periods, called eons. The
two oldest eons are part of what is called
Precambrian time. The third eon, which
includes present time, is called the Phanerozoic
eon. It is divided into three periods
called eras—the Paleozoic, Mesozoic,
and Cenozoic eras.
Geologists learn about these different
periods by studying the many layers of
rock in Earth’s crust. The deepest layers
Geologists set up equipment at Mount Saint
Helens in Washington State.
BRITANNICA STUDENT ENCYCLOPEDIA Geology 97
are the oldest. More recent layers are
closer to the surface. Each layer contains
unique types of rocks and fossils. These
give geologists many clues about Earth’s
history.
#More to explore
Earth • Fossil • Mineral • Prehistoric
Life • Rock
Geometry
Geometry is a kind of mathematics that
deals with shapes and figures. Geometry
explains how to build or draw shapes,
measure them, and compare them.
People use geometry in many kinds of
work, from building houses and bridges
to planning space travel.
Points, Lines, and Angles
Points, lines, and angles are basic terms
of geometry. A point is an object that
has no length or width. It is usually represented
by a dot. A line is an object that
extends without end in both directions.
It is usually drawn with arrowheads to
show that it goes on forever. A line segment
is a part of a line with two end
points. A ray is half of a line, with one
end point.
Two rays with the same end point form
an angle. If the rays are the two halves of
a single line, the angle is called a straight
